There are many kinds of rail road cars for carrying particulate material, be it sand or gravel aggregate, plastic pellets, grains, ores, potash, coal or other granular materials. These materials are not liquid, yet may in some ways tend to flow in a somewhat liquid-like manner. Many of those cars have an upper opening, or accessway of some kind, by which the particulate is loaded, and a lower opening, or accessway, or gate, by which the particulate material exits the car under the influence of gravity. Clearly, while the inlet opening need not necessarily have a movable gate (but may include a cover to discourage contamination of the lading or exposure of the lading to the wind), the outlet opening requires a governor of some kind that is movable between a closed position for retaining the lading while the lading is being transporting, and an open position for releasing the lading at the destination. The terminology “flow through” or “flow through rail road car” or “center flow” car, or the like, may sometimes be used for cars of this nature where lading is introduced at the top, and flows out at the bottom.
Consider, for example, a hopper car for transporting aggregate, be it gravel or sand. The hopper may have a converging hopper discharge section that has the shape, generally speaking, of an inverted four sided, truncated pyramid. At the truncated bottom end, there may be a stationary plate and a moving plate, or door. When the moving plate and the stationary plate are brought together, the door is closed. The car is filled with lading, and is hauled to its destination. At the destination, the gate is opened, and the lading is allowed to escape from the hopper. However, it sometimes happens that, for example, the car may move while the gate is still obstructed by lading, such that the gate may tend to “plow” the aggregate. This may not necessarily lead to the retention of the original geometry of the closure, and, after a time, the gate may tend not to close as well as it might originally have done, or as might be desired. A number of considerations arise from dealing with this kind of issue. First, it may be helpful to diminish, or to avoid, the tendency to distort the geometry of the door closure in the first place. Second, if the door seal region is prone to damage or abuse, it may be helpful to be able to replace the parts most likely to wear or be damaged relatively easily, rather than having to replace what might otherwise be considered permanent structure. Third, it is a consideration that parts employed in this kind of use may face an abrasive environment, even in normal, non-abusive operation. Fourth, particularly if the car is intended to be used with fine aggregates, such as sand, it may be desirable to employ a door seal that may tend to be somewhat tolerant of geometric mismatch, or creeping tolerances as parts are either damaged or bent out of shape.